Dissolving Image Rollover script adds a dissolving effect during image change on mouse over.
The script works across all browsers, though as alluded to, only IE4 will experience the added transition. Just for the uninitiated, a rollover image requires that you specify two images- one while the mouse is not over it, and one for when it is.
This script is in fact rather versatile in the kinds of effects it can render. While the default is "dissolving", 23 other transitions are supported as well, everything from "swipe in", "box out", to "fade in."